It is known that water can be converted into usable fuels. Moreover, it exists in such abundance that it is sometimes viewed as a feedstock for fuels of the future. The photolytic production of hydrogen and oxygen from water is technologically feasible and it is believed to represent the ultimate solution to the world's energy problems. This conversion can be illustrated by the following oxidation and reduction reactions. ##EQU1## wherein a photosensitizer (D) absorbs visible light and thereby reduces an electron relay A to A.sup.- with a concomitant oxidation of D to D.sup.+. Subsequently, the reduced relay A.sup.- releases hydrogen from water while it is simultaneously oxidized to A in a catalyst mediated process. Meanwhile, electrons provided by water reduce the oxidized photosensitizer (D.sup.+) back to D with a simultaneous generation of oxygen.
In copending application Ser. No. 184,460 filed Sept. 5, 1980, now abandoned, which was continued into application Ser. No. 375,913 filed May 7, 1982 we demonstrate that metal oxides can be used to mediate the production of oxygen from aqueous solutions. Specifically, it was demonstrated that the combination of ruthenium oxide in its colloidal or macrodispersed form with colloidal platinum, a sensitizer such as Ru(bipy).sub.3.sup.+2 and an electron relay such as methylviologen mediates the photochemical dissociation of water into hydrogen and oxygen. This system produces hydrogen and oxygen simultaneously as a gaseous mixture thus requiring a separation of the said gases. Moreover, cross reactions add to the cost and complexity of the system.
In copending application Ser. No. 292,966 filed Aug. 14, 1981, as a continuation of application No. 184,610 filed Sept. 5, 1980 the gaseous mixture problem was overcome by generating and collecting the hydrogen and oxygen in separate compartments, that is, in separate halfcells.
This invention provides an improved anode for use in systems comprised of two halfcells or compartments connected by electrodes and a conductive bridge for ion transport. The halfcell containing the anode is operated as a darkened halfcell, that is, it is not necessary to irradiate it with visible light. The halfcell containing the cathode is light permeable and it is operated under visible light irradiation.
One object of this invention is to provide an improved anode for the photolytic production of oxidized substrates. These anodes exhibit a low overvoltage with respect to the substrate which is sought to be oxidized.
This invention demonstrates that light-induced redox reactions occurring in one compartment of the cell system can be coupled to oxidation processes in the other compartment. Conversely, oxidation products formed at the anode are in a stoichiometric relationship to the current which is generated in the electron conductive element.
A further object is the reconversion of the oxidized species (D.sup.+) to its original or reduced form (D). This is achieved via the transfer of electrons from the anodic compartment where substrate oxidation occurs to the cathodic halfcell. This transfer is effected via an external circuit or conductive element which joins one electrode to the other. The recycling of sensitizer (D) lends economy to the process.
The available alternatives on the cathodic side are determined by the nature and function of the sensitizer (electron donor) and electron relay (electron acceptor).
The sensitizer provides three possibilities. It may be present in the cathodic compartment as a dissolved species in solution; it may be adsorbed on the cathode surface or, in the third situation, the sensitizer may be present as a photoexcitable semiconductor material which functions per se as the cathode. Suitable semiconductors include, for example, p-type gallium phosphide (p-GaP), gallium arsenide and silicon.
The electron relay may be a sacrificial species which is irreversibly reduced during the photo-redox process or it may be regenerated and continuously recycled.
In a process which provides for regeneration the sensitizer and relay may be coupled catalytically to the Noble metals as, for examle, colloidal platinum, ruthenium, palladium, rhodium, gold and silver are particularly suitable for this purpose.
The selection of a suitable anode for the anodic side of the system is critical to the process. Specifically, we have discovered that overall cell efficiency is enhanced by utilizing an anode which possesses a low over-voltage with respect to the substance which is sought to be oxidized. Anodes found suitable for this purpose are electrocatalyst anodes, i.e., anodic catalysts such as (a) ruthenium oxide, (b) iridium oxide, (c) ruthenate salts, (d) iridate salts, and mixtures of two or more thereof. The anodic catalyst may optionally also include (e) transition metal oxides, (f) rare earth metal oxides, (g) aluminum oxide, (h) silicon oxide and (i) mixtures of two or more thereof. In compositions comprising the oxides (e)-(i) the ruthenium and iridium components (a)-(d) generally constitute no less than about 50% by weight of the total composition and, preferably, no less than about 75% by weight.
The rughenium oxides of this invention are compounds of the formula: RuOx wherein x is an integer having a value of 1.5-2.0.
Transition metal oxides and rare earth metal oxides which may be used in combination with the oxides and salts of ruthenium and iridium include, for example, the oxides of tungsten, zirconium, tantalum, titanium, chromium, vanadium, iron, nickel, cobalt and manganese. Preferred among these transition metals are di-tantalum pentoxide and zirconium oxide.
In addition to the electrocatalyst functioning per se as the anode this invention includes anodes in which the said electrocatalyst is coated as a layer on a conductive support. Typical of such anodes are, for example, titanium or platinum coated with ruthenium containing compositions such as ruthenium oxides, ruthenates, lanthanum ruthenate (LaRuO.sub.3) and lead ruthenate (PbRuO.sub.3), iridium containing compositions such as iridates, for example, lanthanum iridates or lead iridates (LaIrO.sub.3, PbIrO.sub.3) or iridium oxides and mixtures of ruthenium oxide or iridium oxide with transition metal oxides such as titanium or tantalum oxides such as ditantalum pentoxide or zirconium oxide and the like.
Other conductive materials which may be used as a support for the oxides and salts of ruthenium and iridium are graphite silica and the oxides of alumina, chromia, chloria and thorium or mixtures of same, with the proviso that at least one of the oxides and salts of ruthenium and iridium are present in the combination.
The coated anodes are prepared by depositing a 1-5 .mu.m layer of the electrocatalyst on the desired support, although a 2-4 .mu.m layer is deemed economically preferable. Thus, according to one aspect of this invention a particularly suitable anode was prepared by depositing a thin 2 .mu.m layer of ruthenium oxide on a 0.3 mm thick plate of titanium having a total surface area of 8 cm.sup.2.